Air bags which inflate upon impact to protect passengers in vehicles have become standard safety items. Their inflators must be inherently stable for long shelf life, and readily and reliably be initiatiable so as to generate and release a substantial volume of gas at a prescribed pressure very quickly, in fact in small fractions of a second.
It is known to store gas for this purpose at a high pressure to release upon impact. The long-term storage of high pressure gas, and the consequences of an unintended breach of its container are distinct drawbacks to such arrangements. Instead the preference is to utilize a chemical charge which, when initiated will release gas in the desired volume and at the requisite pressure. Currently the preference for such a charge is a solid. The known solids generally generate gases and residues which are toxic, or at best are environmentally undesirable.
The disadvantages of both of these charges are well-known. The gas bottle is carried as a potential risk both during the lifetime of the automobile and when the vehicle is scrapped. If the wrecker has not removed the bottle, and inadvertently cuts it, an explosive situation could occur without his foreknowledge. It is both a physical threat of flying objects, and/or a fire threat, because its contents may catch on fire.
The solid charges involve the risks to the wrecker that he may initiate the charge without knowing it, incurring exposure to a toxic substance or of spilling a dangerous substance. The solid products are generally not environmentally suitable, and can readily be ignited.
Further, there are substantial risks inherent in the manufacture of the solid propellent, such as are commonly used for solid gas-generating charges. These charges, often azides, are tabletted in munitions-grade facilities, and have their share of accidents, many of them fatal. The very property that makes these products suitable for air bag inflation--quick reaction to generate high temperatures gases--is also the property which makes them risky to process.
It has only recently become appreciated that a liquid charge can be even more effective for gas generation than a solid charge, and that some of the above risks inherent in the use of a solid charge and compressed gases can be overcome. In fact an inflator according to this invention can eliminate all of them.
However, the earlier efforts to use liquid charges that are known to the inventors herein have required complicated apparatus, and chemical ignition charges that are initiated by still another initiator to set them off. Movable gas control means in them such as pistons are the norm to control the gas-generation reaction and to keep it "burning".
For example, Giovanetti U.S. Pat. No. 5,060,973 utilizes a liquid propellant--a hydroxyl ammonium nitrate-based propellant, which is ignited by a pyrotechnic igniter such as, smokeless powder set off by an electric primer like an exploding bridge wire. In order to ignite the gas-generating charge in the device, the products of combustion of the pyrotechnic igniter must be discharged into it, and in a controlled manner. This requires a movable piston to separate the main charge from the igniter charge, and separate channel means to convey the ignited gases into a reaction chamber while the piston gradually forces the main charge into the reaction chamber.
This is a relatively complicated arrangement, and friction, degradation or destruction of the piston would be expected to lead to an uncertain output. Furthermore, the propellant compound if it leaked (during scrapping of the vehicle, for example) would contaminate the surroundings. In addition, the black powder charge would still be present as a risk to setting off the main charge.
Brede 5,030,730 utilizes a liquified gas--a short chain hydrocarbon such as propane, in combination with nitrous oxide (N20). To ignite this charge, a pyrotechnic igniter is also used to propel a penetrator piston to puncture the container. The piston carries two sets of channels. One is to carry igniter gas into the storage chamber, and the other is to carry them to a chamber from which the hot gases enter the air bag. In some of his embodiments he keeps his components in separate chambers, and the piston has the additional function of enabling the two components to mix. Then the piston is also required to pierce the two chambers.
The hazards that accompany a stored charge of liquefied gas under pressure have been described above, and are found in this construction. Also it requires a movable piston to open the storage chamber or chambers and to regulate and direct the flow of the liquids. Such complications are undesirable in a critical man-safety device.
It is the object of this invention to provide an inflator which requires no moving parts, which even though its charge is under moderate pressure constitutes no storage or scrapping risk, which does not require pyrotechnic means for the charge itself or for its ignition, whose charge composition is environmentally benign, and which can be manufactured, stored and scrapped with no or negligible risk to equipment, facilities, or human life.
The charge of this invention is a liquid monopropellant. As the term "monopropellant" is used herein it means a compound or a mixture of compounds which are stored mixed together, and which, when initiated, involves no addition of other substances to generate the gases.
The use of a liquid monopropellant offers advantages not readily attainable with solid charges and/or hybrid inflators. For example by modifications of the chamber or chambers in which the liquid is stored and combusted (which often is the same chamber), an inflator can be adapted to produce gases at different temperatures, pressures, flow rates, rates of pressure rise, and multiple pulses.
The structures involved are elegantly simple and are highly reliable. It is even possible to adapt such structures to produce sequential pulses of gas triggered by a second impact, or to accommodate such variables as heavier passengers who might require a higher rise rate of gas pressure.
It is another object of this invention to provide such an inflator whose initiator means is unsensitive to stray electrical charges or interference.
It is another object of this invention to provide an inflator with the foregoing advantages among others.